Pre-carburized molybdenum-modified zeolite catalyst and use thereof for the aromatization of lower alkanes

ABSTRACT

The present invention relates to a method for producing a zeolite catalyst useful for aromatization of a lower alkane, a zeolite catalyst useful for aromatization of a lower alkane obtainable by said method and a process for aromatization of a lower alkane using the zeolite catalyst of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent Ser. No. 13/697,640,Filed Nov. 13, 2012, now U.S. Pat. No. 9,266,100, which is a 371 ofInternational Application No. PCT/EP2011/002435, filed May 17, 2011,which claims priority to European Application No. 10005263.8, filed May20, 2010, which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a method for producing amolybdenum-modified zeolite catalyst useful for aromatization of a loweralkane, a zeolite catalyst useful for aromatization of a lower alkaneobtainable by said method and a process for aromatization of a loweralkane using the zeolite catalyst of the present invention.

BACKGROUND ART

It has been previously described that lower alkanes can be directlyconverted into higher hydrocarbons using a molybdenum-modified mediumpore-size zeolite catalyst.

Wang (1993) Catal Lett 21, 35-41, for instance, describes the catalyticconversion of methane into benzene under non-oxidizing conditions usinga ZSM-5 zeolite catalyst on which molybdenum has been deposited.

A major drawback of the use of molybdenum-modified medium pore-sizezeolite catalyst for the aromatization of lower alkanes is that coke isdeposited on the catalyst surface which quickly reduces catalystactivity.

Shu (2002) Chemistry Letters, 418-419 describes that de-alumination byacid reflux of Mo-loaded zeolites like ZSM-5 and MCM-22 leads todecreased coke formation in methane aromatization reactions. Shu teachesthat this reduction in coke formation is caused by a reduction of thenumber of Brønsted acid sites on the catalyst surface.

WO 02/10099 describes that the catalyst activity in a methanearomatization process can be stabilized by activating themolybdenum-loaded ZSM-5 catalyst with a combined stream which comprisesover 25 mole-% of hydrogen and methane prior to contacting the catalystwith the methane feed.

DISCLOSURE OF INVENTION

It was an object of the present invention to provide a further improvedprocess for converting lower alkanes into aromatics. This is achieved byproviding the subject matter as described herein below and ascharacterized in the claims.

Accordingly, the present invention provides a method for producing azeolite catalyst useful for aromatization of a lower alkane comprising:

(a) providing a zeolite catalyst precursor comprising 2-10 wt %molybdenum (Mo) and 0-2 wt % of one or more additional elements selectedfrom Groups 6-11 of the Periodic Table; and

(b) contacting the provided zeolite catalyst precursor with apre-carburizing gas stream comprising a lower alkane and 50-90 mole-% ofan inert diluent gas at a temperature that is gradually increased from20-250° C. to the temperature useful for aromatization and keeping thetemperature constant for 0-60 minutes at the temperature useful foraromatization.

In the context of the present invention, it was found that thepre-carburization of Mo-loaded H-ZSM-5 zeolite catalyst precursor with acombined stream of the lower alkane methane and inert diluent gas likenitrogen at a constantly increasing temperature e.g. from 100° C. to thetemperature useful for aromatization (750° C.) remarkably improves thestability/performance of the catalyst for methane aromatization.Moreover, it was found that catalyst performance is even furtherimproved in case the catalyst precursor is pre-carburized under thecombined stream of methane and nitrogen at a constantly increasingtemperature to the temperature useful for aromatization and issubsequently kept for e.g. 15 minutes at the temperature useful foraromatization.

Accordingly, the zeolite catalyst produced by the method of the presentinvention is useful in a process for converting a feedstream comprisinga lower alkane to a product stream comprising aromatic hydrocarbons.This process for converting a lower alkane to aromatic hydrocarbons isalso described herein as “lower alkane aromatization”. Preferably, the“lower alkane” is methane (CH₄), ethane (C₂H₆) or a mixture thereof.Preferably, said mixture comprises up to 20 mole-% ethane in methane.Most preferably, the “lower alkane” is methane (CH₄). The aromatichydrocarbons produced by the present lower alkane aromatization processinclude benzene, toluene and xylenes (commonly denoted as “BTX”).

The term “pre-carburizing gas stream” as used herein relates to a gasstream comprising a lower alkane and 50-90 mole-% of an inert diluentgas. The term “inert diluent gas” as used herein relates to an elementor compound (or a mixture thereof) which is gaseous at the conditionsused for pre-carburization and which does not participate in and/oradversely interfere with the chemical reactions that occur when thecatalyst is contacted with the pre-carburizing gas stream duringpre-carburization. Preferably, the inert diluent gas is selected fromthe group consisting of nitrogen (N₂), helium (He) and argon (Ar).Hydrogen (H₂), for instance, is not an “inert diluent gas” since it isknown to act as a reducing agent when comprised in the pre-carburizinggas stream. Accordingly, the “inert diluent gas” of the presentinvention does not comprise H₂ or other reducing components. The maximumallowable amount of other components like reducing components in thepre-carburizing gas stream is 10 mole-%, preferably up to 5 mole-% andmore preferably up to 2 mole-%. Most preferably, the pre-carburizing gasstream consists essentially of lower alkane and inert diluent gas (i.e.less than 1 mole-% of other components).

In the method of the present invention, the temperature is graduallyincreased from room temperature (i.e. about 20° C.)—250° C. to thetemperature useful for aromatization. Preferably, the temperature isgradually increased from 100-250° C. to the temperature useful foraromatization. The temperature useful for aromatization can be easilydetermined by the person skilled in the art; see e.g. Ismagilov (2008)Energy and Environmental Science 526-541. Preferably, the temperatureuseful for aromatization is 600-850° C., more preferably 700-750° C. Thepressure at which the aromatization reaction of the present inventioncan be carried out can be easily determined by the skilled person andpreferably is 0.2-5 atmosphere, more preferably 0.5-2 atm.

The term “gradually increased temperature” or “temperature that isgradually increased” as used herein means that the temperature isincreased at a predetermined rate over a period of time. In the methodaccording to the invention, the temperature is preferably increased at arate of about 20° C./min or less, more preferably at a rate of about 10°C./min or less and most preferably at a rate of about 5° C./min.

When the temperature useful for aromatization is reached after graduallyincreasing said temperature starting from 20-250° C., the temperaturemay be kept constant for a certain period of time before, for instance,switching the gaseous feed of the catalyst from the pre-carburizationstream to a feedstream for aromatization. Preferably, the temperature iskept constant for 5-60 minutes at the temperature useful foraromatization after attaining said temperature useful for aromatization.Most preferably the temperature is kept constant for 15 minutes at thetemperature useful for aromatization after attaining said temperatureuseful for aromatization. It is preferred to start aromatizationreaction immediately after the pre-carburization. However, it ispossible to cool down the catalyst after pre-carburization and then tolater directly use the catalyst without having to redo thepre-carburization. In such cases time and carrier gas may be wasted and,hence, the efficiency of the process will decrease.

The term “zeolite catalyst precursor” or “catalyst precursor” as usedherein relates to the zeolite-based composition at any stage prior tothe pre-carburizing step (b) as described herein above.

Prior to the pre-carburization step, the zeolite catalyst precursor ofthe present invention comprises 2-10 wt % Mo, preferably 3-5 wt % Mo. Inaddition thereto, the zeolite catalyst precursor may further contain upto 2 wt %, preferably up to 0.5 wt % of one or more additional elementsselected from Group 6-11 of the Periodic Table (IUPAC version of 22 Jun.2007). In one embodiment, said one or more additional elements that maybe comprised in the catalyst precursor are selected from Group 6-10 ofthe Periodic Table. Preferred additional elements are selected from thegroup consisting of tungsten (W), platinum (Pt), ruthenium (Ru), rhenium(Re), cobalt (Co), copper (Cu) and iron (Fe). Methods useful fordetermining the quantity of Mo and other elements comprised in thecompositions as described herein are well known in the art and includeAAS (Atomic Absorption Spectrometer) or ICP (Inductively Coupled PlasmaSpectrometry) analysis.

Microporous aluminosilicate zeolites useful in a process for loweralkane aromatization are well known in the art. Preferably the zeoliteis a medium-pore size zeolite having a pore size of about 5-6 Å.Suitable medium-pore size zeolites are 10-ring zeolites. i.e. the poreis formed by a ring consisting of 10 SiO₄ tetrahedra. In one preferredembodiment, the zeolite is of the pentasil type. Most preferably, thezeolite is H-ZSM-5. Other zeolites known to be useful for lower alkanearomatization include, but are not limited to MCM-22 and H-ZSM-11.

It is preferred that the zeolite is in the hydrogen form: i.e. having atleast a portion of the original cations associated therewith replaced byhydrogen. Methods to convert an aluminosilicate zeolite to the hydrogenform are well known in the art. A first method involves direct ionexchange employing an acid. A second method involves base exchangefollowed by calcination.

The zeolite of the present invention may be dealuminated. Accordingly,the zeolite preferably has a Si/Al ratio of 10-50. Means and methods toobtain dealuminated zeolite are well known in the art and include, butare not limited to the acid leaching technique; see e.g. Post-synthesisModification I; Molecular Sieves, Volume 3; Eds. H. G. Karge, J.Weitkamp; Year (2002); Pages 204-255. In the context of the presentinvention it was found that using a dealuminated H-ZSM-5 zeolite havinga Si/Al ratio of 10-50 improves the performance/stability of thecatalyst. Means and methods for quantifying the Si/Al ratio of adealuminated zeolite are well known in the art and include, but are notlimited to AAS (Atomic Absorption Spectrometer) or ICP (InductivelyCoupled Plasma Spectrometry) analysis.

The zeolite catalyst precursor that is subjected to thepre-carburization method of the present invention may be produced by anyconventional method. Preferably, the provided zeolite catalyst isproduced by a method comprising depositing Mo and optionally one or moreadditional elements selected from Group 6-11 of the Periodic Table onthe zeolite using an incipient wetness method which comprises the stepsof contacting a zeolite with a solution comprising a soluble Mo-salt andoptionally a solution comprising one or more additional elementsselected from Group 6-11 of the Periodic Table; and drying the zeoliteto provide a zeolite catalyst precursor. Deposition of metal(s) onto thezeolite can also be carried out by using impregnation technique inaqueous solution under acidic as well as basic conditions. In oneembodiment, said one or more additional elements that may be depositedare selected from Group 6-10 of the Periodic Table.

The Mo and one or more additional elements may be deposited on thezeolite concurrently by contacting the zeolite with a solutioncomprising both a soluble Mo-salt and (a) soluble salt(s) comprising oneor more additional elements selected from Group 6-11 of the PeriodicTable. Alternatively, the Mo and one or more additional elements may bedeposited on the zeolite subsequently by contacting the zeolite with asolution comprising Mo and a different solution comprising one or moreadditional elements selected from Group 6-11 of the Periodic Table. Whenone or more additional elements are deposited, it is preferred that Mois deposited first. The solution used for depositing the Mo and saidoptional additional element(s) preferably is an aqueous solution. In oneembodiment of the present invention, the zeolite catalyst precursor isdried in air.

After drying, the catalyst precursor on which Mo and the optionaladditional element(s) are deposited is calcined in air, preferably inmoisture free air. Preferably, the catalyst precursor is calcined at500-650° C. and a pressure of 1 atm for 1-5 hrs. Most preferably thecatalyst precursor is calcined at 600° C. for 2 hrs.

Accordingly, a method for producing a zeolite catalyst useful foraromatization of a lower alkane is provided comprising:

(a) contacting a zeolite with a solution comprising molybdenum (Mo) andoptionally a solution comprising one or more additional elementsselected from Group 6-11 of the Periodic Table;

(b) drying and calcining the product of step (a) to provide a zeolitecatalyst precursor comprising 2-10 wt % molybdenum (Mo) and 0-2 wt % ofone or more additional elements selected from Group 6-11 of the PeriodicTable; and

(c) contacting the provided zeolite catalyst precursor with apre-carburizing gas stream comprising a lower alkane and 50-90 mole-% ofan inert diluent gas at a temperature that is gradually increased from20-250° C. to the temperature useful for aromatization and keeping thetemperature constant for 0-60 minutes at the temperature useful foraromatization.

Preferably, the zeolite catalyst prepared by the method of the presentinvention and which is useful in a process for the aromatization of alower alkane further comprises a binder. Preferably, the binder isselected from the group consisting of lanthanum-exchanged Kaolin andalpha alumina and most preferably the binder is La-exchanged kaolinbinder. In the context of the present invention it was found that theuse of binder improves the catalyst performance.

Accordingly, the dried and calcined zeolite catalyst precursorcomposition may subsequently be bound with a binder before subjectingsaid catalyst precursor to pre-carburization. Therefore, the catalystand the binder are thoroughly mixed, preferably at a weight ratio of4-1:1, most preferably at a weight ratio of about 2:1. The catalystmixture may then be formed into pellets, for instance by pressing themixed catalyst and binder composition at e.g. 10 tons of pressure. Thepressed catalyst composition may subsequently be crushed and sieved toprovide zeolite catalyst precursor in particulate form. The crushedsolids containing particle sizes from 0.5 to 1.0 mm are preferablyselected for catalytic use.

In a further embodiment, a zeolite catalyst useful for aromatization ofa lower alkane is provided that is obtainable by the method forproducing a zeolite catalyst of the present invention.

In yet another embodiment of the invention, a process for aromatizationof a lower alkane is provided comprising contacting the catalyst of thepresent invention with a feedstream comprising a lower alkane atconditions useful for aromatization. Accordingly, the present inventionprovides a process for the aromatization of a lower alkane comprising:producing a zeolite catalyst useful for aromatization of a lower alkaneaccording to the method of the present invention; and contacting saidcatalyst with a feedstream comprising a lower alkane at conditionsuseful for aromatization. Preferably, the aromatization process of thepresent invention is performed at non-oxidizing conditions.

As used herein, the term “feedstream” relates to the gaseous streamwhich is brought into contact with the catalyst to convert the thereincomprised lower alkane into aromatic hydrocarbons. In one embodiment,the feedstream is different from the pre-carburizing gas stream in thatit e.g. does not comprise an inert diluent. Preferably, the feedstreamconsists of lower alkane, more preferably 0-20 mole-% ethane in methaneand most preferably, the feedstream consists of pure methane.

The conditions useful for aromatization can be easily determined by theperson skilled in the art; see e.g. Ismagilov (2008) Energy andEnvironmental Science 526-541. Again, the temperature useful foraromatization may be 600-850° C. and preferably 700-750° C. Furthermore,the aromatization reaction as described herein preferably is performedat a WHSV 0.1-2 h⁻¹ and/or a pressure 0.2-5 atm.

MODE(S) FOR CARRYING OUT THE INVENTION

The present invention will now be more fully described by the followingnon-limiting Examples.

Comparative Example 1 Non-Pre-Carburized 3.5% Mo H-ZSM-5

In order to prepare Mo/H-ZSM-5 zeolite, 0.65 g ammonium molybdatetetrahydrate was dissolved in 15 ml demineralised water. 10 g of powderH-ZSM-5 having Si/Al ratio of 11.5 was added to the above solution. Theresulting paste was thoroughly mixed and dried at 100° C. for 12 h. Thedried catalyst mass was further heated up to 600° C. at a rate of 5°C./min followed by calcined at 600° C. for 2 h in the presence ofmoisture-free air.

To prepare the binder, 0.19 g lanthanum nitrate hexahydrate wasdissolved in 120 ml demineralised water. Subsequently, 6 g of powderkaolin was added to the above solution. The mixture was heated at 95-98°C. for 24 h under continuous stirring. Resulting La-exchanged kaolin wasseparated by filtration. The retained solid mass was washed with 2liters of demineralised water and dried at 100° C. for 12 h. DriedLa-exchanged kaolin was calcined in a muffle furnace at 650° C. for 4 hunder flowing moisture-free air (flow: 100 ml/min) The solid mass wasthen cooled to room temperature. La content in the binder material wasdetermined by AAS (Atomic Absorption Spectrometer) to be 1 wt. % La onKaolin.

The catalyst compositions comprising of Mo-containing H-ZSM-5 basedzeolites and La-exchanged kaolin binder were prepared in particle formby mixing thoroughly the catalyst and the binder in the ratio of 2:1.The catalyst mixture was then pressed at 10 ton pressure to makepellets. The pressed catalyst compositions were crushed and sieved. Thecrushed solids containing particle sizes from 0.5 to 1.0 mm wereselected for catalytic use.

2.0 g catalyst particles were loaded in a down flow fixed bedmicro-catalytic reactor. The temperature of the reactor is heated underconstant N₂ flow. After attaining the temperature of the reactor to thedesired reaction temperature (750° C.), N₂ flow is stopped and a puremethane flow is fed to the catalyst bed (20 ml/min at 1 atm) and thereaction started. The Weight Hourly Space Velocity (WHSV) was 0.4 h⁻¹.Unconverted methane and the products formed were analysed by an on-lineGas Chromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 1.

TABLE 1 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 14.5 11.4 64.8 67.5 21.1 2 13.5 13.071.6 74.7 12.3 3 12.7 14.2 73.8 77.2 8.6 4 11.4 16.4 74.6 78.1 5.5 510.2 18.4 75.7 79.0 2.6 6 8.6 21.0 75.7 78.5 0.5 7 7.5 24.0 73.4 75.70.3 8 6.8 27.4 70.3 72.4 0.2 9 6.1 30.1 67.7 69.7 0.2

Comparative Example 2 Non-Pre-Carburized 3.5% Mo/Dealuminated H-ZSM-5

The experimental procedures for comparative Example 2 were identical toComparative Example 1 with the exception that dealuminated H-ZSM-5zeolite was used.

The Mo-modified dealuminated H-ZSM-5 zeolite of Comparative Example 2was prepared as follows. 0.65 g ammonium molybdate tetrahydrate wasdissolved in 15 ml demineralised water. 10 g of powder dealuminatedH-ZSM-5 having Si/Al ratio of 12.6 was added to the above solution. Theresulting paste was thoroughly mixed and dried at 100° C. for 12 h. Thedried catalyst mass was further heated up to 600° C. at a rate of 5°C./min followed by calcined at 600° C. for 2 h in the presence ofmoisture-free air.

10 g parent H-ZSM-5 having Si/Al ratio of 11.5 was dispersed in 200 mlof aqueous 6 (N) nitric acid solution in a round bottom flask. Themixture was heated at 95-100° C. under stirring for 5 h. The solid masswas filtered out and washed thoroughly with 2 liters of demineralisedwater and dried at 100° C. for 12 h. The Si/Al ratio of the zeolite wasdetermined by AAS (Atomic Absorption Spectrometer) to be 12.6.

The same process conditions as in Comparative Example 1 were used.Unconverted methane and the products formed were analysed by an on-lineGas Chromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 2.

TABLE 2 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 15.7 11.0 55.3 57.7 31.3 2 14.7 12.665.0 67.9 19.5 3 13.2 14.4 70.7 74.0 11.6 4 12.1 15.9 72.7 76.1 8.0 510.9 18.3 74.0 77.4 4.3 6 9.9 20.4 74.3 77.6 2.0 7 8.9 22.8 73.2 76.21.0 8 8.1 26.0 70.7 73.4 0.6 9 7.4 28.2 69.1 71.5 0.3

Comparative Example 3

3.5% Mo/dealuminated H-ZSM-5 pre-carburized with pure methane atconstantly increasing temperature from 100-700° C. and holding time of0.25 h at 700° C.

The 3.5% Mo/dealuminated H-ZSM-5 of Comparative Example 3 was preparedas described under Comparative Example 2.

In comparative Example 3, however, the catalyst was first subjected topre-carburization using a pre-carburization gas stream consisting ofpure methane and thus which does not comprise an inert diluent.

Therefore 2.0 g catalyst particles were loaded in a down flow fixed bedmicro-catalytic reactor and pre-carburized in the following way:

Step 1: Exposed to the flowing moisture-free N₂ (flow: 25 ml/min) at100° C. for 0.25 h.

Step 2: Exposed to the moisture-free stream consisting of pure methane(20 ml/min) under a constantly increasing temperature ramp of 5° C./minfrom 100° C. to 700° C., followed by holding at that temperature for0.25 h at 700° C.

Step 3: Exposed to a moisture-free N₂ (flow: 50 ml/min) at 700° C.,followed by increasing the temperature from the pre-carburizationtemperature to 750° C. using the ramp 5° C./min.

After pre-carburization of catalyst and attaining the temperature of thereactor to the desired reaction temperature (750° C.), N₂ flow isstopped and the methane flow fed to the catalyst bed is set at 20 ml/minat 1 atm and the reaction started. The Weight Hourly Space Velocity(WHSV) was 0.4 h⁻¹. Unconverted methane and the products formed wereanalysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2column, using a Flame Ionization Detector. The obtained results aresummarized in Table 3.

TABLE 3 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 14.2 12.4 63.6 66.2 21.4 2 13.6 14.069.8 72.6 13.4 3 13.0 14.4 70.9 74.0 11.6 4 12.2 15.7 71.4 74.8 9.5 511.3 16.9 71.1 74.6 8.5 6 10.4 19.1 71.2 74.5 6.4 7 9.6 22.1 69.4 72.45.5 8 8.8 25.3 68.9 71.8 2.9 9 8.1 28.4 68.0 70.6 1.0

Comparative Example 4

3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and H₂ atconstantly increasing temperature from 100-700° C. and holding time of0.25 h at 700° C.

Comparative Example 4 is identical to Comparative Example 3, with theexception that the pre-carburizing gas stream consists of methane (10ml/min) and H₂ (10 ml/min) Accordingly, the pre-carburizing gas streamdoes not comprise an inert diluent.

The same process conditions as in Comparative Example 3 were used.Unconverted methane and the products formed were analysed by an on-lineGas Chromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 4.

TABLE 4 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 13.9 11.0 59.2 62.7 26.3 2 13.7 11.762.8 65.6 22.7 3 13.2 12.5 66.8 70.0 17.5 4 12.4 13.7 69.4 72.8 13.5 511.6 15.5 71.2 74.6 9.9 6 10.3 17.6 71.9 75.1 7.3 7 9.6 19.9 72.3 75.34.8 8 8.8 21.6 72.0 74.9 3.5 9 7.7 22.8 71.4 74.4 2.8

Comparative Example 5

3.5% Mo/dealuminated H-ZSM-5 pre-carburized at 750° C. for 15 min withmethane and N₂

The 3.5% Mo/dealuminated H-ZSM-5 of Comparative Example 5 was preparedas described under Comparative Example 2.

In Example 5, however, the catalyst was subjected to pre-carburizationusing a pre-carburization gas stream containing methane and N₂.Therefore, 2.0 g catalyst particles were loaded in a down flow fixed bedmicro-catalytic reactor and pre-carburized in the following way:

Step 1: Exposed to the flowing moisture-free N₂ (flow: 25 ml/min) at100° C. for 0.25 h, followed by increase temperature to 750° C. under aconstantly increasing temperature range of 5° C./min.

Step 2: Exposed to the moisture-free stream containing methane (10ml/min) and N₂ (30 ml/min) at 750° C. for 0.25 h. Hence, the temperaturewas not gradually increased during pre-carburization.

Step 3: Exposed to a moisture-free N₂ (flow: 50 ml/min) at 750° C. for0.1 h.

After pre-carburization of catalyst and attaining the temperature of thereactor to the desired reaction temperature (750° C.), N₂ flow isstopped and the methane flow fed to the catalyst bed is set at 20 ml/minat 1 atm and the reaction started. Pure methane was used as a feedstreamfor the reaction. The Weight Hourly Space Velocity (WHSV) was 0.4 h⁻¹.Unconverted methane and the products formed were analysed by an on-lineGas Chromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 5.

TABLE 5 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 14.6 12.5 67.9 70.6 16.9 2 13.8 12.771.8 74.8 12.5 3 13.4 13.9 71.1 74.4 11.7 4 12.3 15.2 71.7 75.2 9.6 511.6 16.8 70.6 74.3 8.9 6 10.4 18.8 71.2 75.3 5.9 7 9.8 20.2 71.5 75.54.3 8 9.4 22.4 70.1 74.1 3.5 9 8.7 23.9 69.8 73.8 2.3

Example 1

3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and N₂ atconstantly increasing temperature from 100-750° C. with no holding timeat 750° C.

Example 1 is identical to Comparative Example 5 with the exception thatthe catalyst was pre-carburized in the following way:

Step 1: Exposed to the flowing moisture-free N₂ (flow: 25 ml/min) at100° C. for 0.25 h.

Step 2: Exposed to the moisture-free stream containing methane (10ml/min) and N₂ (30 ml/min) under a constantly increasing temperatureramp of 5° C./min from 100° C. to 750° C. Hence, the temperature wasgradually increased during pre-carburization.

Step 3: Exposed to a moisture-free N₂ (flow: 50 ml/min) at 750° C. for0.1 h.

Subsequently, the catalyst is switched to the methane feedstream. Thesame process conditions as in the comparative Example 1 were used.Unconverted methane and the products formed were analysed by an on-lineGas Chromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 6.

TABLE 6 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 15.0 11.6 65.1 67.9 20.5 2 14.4 12.470.7 73.7 13.9 3 13.3 13.6 75 78.2 8.2 4 12.3 15.4 76 79.2 5.4 5 11.417.0 75.8 78.9 4.1 6 10.6 17.5 76.7 79.8 2.7 7 9.8 19.1 75.4 78.7 2.2 89.5 21.1 73.9 77.1 1.8 9 8.8 23.0 72.4 75.6 1.4

Example 2

3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and N₂ atconstantly increasing temperature from 100-750° C. with holding time of0.25 h at 750° C.

Example 2 is identical to Example 1 with the exception that the catalystwas pre-carburized in the following way:

Step 1: Exposed to the flowing moisture-free N₂ (flow: 25 ml/min) at100° C. for 0.25 h.

Step 2: Exposed to the moisture-free stream containing methane (10ml/min) and N₂ (30 ml/min) under a constantly increasing temperatureramp of 5° C./min from 100° C. to 750° C. followed by holding at 750° C.for 0.25 h.

Step 3: Exposed to moisture-free N₂ (flow: 50 ml/min) at 750° C. for 0.1h.

The same process conditions as in Example 1 were used. Unconvertedmethane and the products formed were analysed by an on-line GasChromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 7.

TABLE 7 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 15.0 11.9 67.8 70.9 17.2 2 14.5 12.471.4 74.7 12.9 3 13.6 14.2 72.3 75.7 10.1 4 13.1 15.3 72.9 76.3 8.4 512.4 16.8 73.3 76.8 6.4 6 11.6 18.1 73.1 76.7 5.2 7 10.9 19.2 72.7 76.44.4 8 10.4 20.7 71.9 75.6 3.7 9 10.1 21.9 71.5 75.2 2.9

Example 3

3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and N₂ atconstantly increasing temperature from 200-750° C. with holding time of0.25 h at 750° C.

Example 3 is identical to Example 1 with the exception that the catalystwas pre-carburized in the following way:

Step 1: Exposed to the flowing moisture-free N₂ (flow: 25 ml/min) at200° C. for 0.25 h.

Step 2: Exposed to the moisture-free stream containing methane (10ml/min) and N₂ (30 ml/min) under a constantly increasing temperatureramp of 5° C./min from 200° C. to 750° C. followed by holding at 750° C.for 0.25 h.

Step 3: Exposed to moisture-free N₂ (flow: 50 ml/min) at 750° C. for 0.1h.

The same process conditions as in Example 1 were used. Unconvertedmethane and the products formed were analysed by an on-line GasChromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 8.

TABLE 8 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 14.1 11.2 67.8 71.0 17.8 2 13.8 12.371.3 74.3 13.4 3 12.8 13.8 74.1 77.2 9.0 4 12.4 14.4 72.3 75.5 10.1 511.5 15.6 73.3 76.6 7.8 6 10.7 17.6 72.5 75.9 6.5 7 10.2 18.2 72.1 75.76.1 8 9.8 19.7 70.8 74.6 5.7 9 9.5 21.5 69.3 73.2 5.3

Example 4

3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and N₂ atconstantly increasing temperature from 100-700° C. with holding time of0.25 h at 700° C.

Example 4 is identical to Example 1 with the exception that the catalystwas pre-carburized in the following way:

Step 1: Exposed to the flowing moisture-free N₂ (flow: 25 ml/min) at100° C. for 0.25 h.

Step 2: Exposed to the moisture-free stream containing methane (10ml/min) and N₂ (30 ml/min) under a constantly increasing temperatureramp of 5° C./min from 100° C. to 700° C. followed by holding at 700° C.for 0.25 h.

Step 3: Exposed to moisture-free N₂ (flow: 50 ml/min) at 700° C.,followed by increasing the temperature from 700° C. to 750° C. using theramp 5° C./min.

The same process conditions as in Example 1 were used. Unconvertedmethane and the products formed were analysed by an on-line GasChromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 9.

TABLE 9 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 15.1 11.4 63.1 65.8 22.8 2 14.5 12.670.2 73.3 14.1 3 13.6 13.6 73.9 77.1 9.3 4 12.1 15.8 76.5 79.8 4.4 511.4 17.5 76.9 80.1 2.4 6 10.6 18.5 77.4 80.5 1.0 7 10.2 20.1 76.3 79.30.6 8 9.4 21.9 74.7 77.7 0.4 9 8.9 23.6 73.3 76.2 0.2

Comparative Example 5

3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and H₂ atconstantly increasing temperature from 100-750° C. and holding time of0.25 h at 750° C.

Comparative Example 5 is identical to Example 2, with the exception thatthe pre-carburizing gas stream consists of methane (10 ml/min) and H₂(30 ml/min) Accordingly, the pre-carburizing gas stream does notcomprise an inert diluent.

The same process conditions as in Example 2 were used. Unconvertedmethane and the products formed were analysed by an on-line GasChromatograph equipped with Petrocol DH 50.2 column, using a FlameIonization Detector. The obtained results are summarized in Table 10.

TABLE 10 Methane Product distribution—Selectivity (wt %) Time/h Conv./%C2-C5 Benzene BTX C9+ aromatics 1 14.1 15.6 63.1 65.4 19.0 2 13.9 16.168.0 70.1 13.8 3 13.5 16.3 71.9 73.1 10.6 4 12.9 16.7 72.3 74.0 9.3 512.4 17.5 73.7 74.9 7.6 6 11.5 18.1 73.8 75.8 6.1 7 10.5 18.8 74.6 76.54.7 8 9.6 19.8 74.0 76.4 3.8 9 8.8 21.3 73.7 76.0 2.7

By comparing the results in Tables 1-5 with Table 6 and by comparing theresults in Table 10 with Table 7, it is clear that the pre-carburizationof Mo-loaded H-ZSM-5 zeolite catalyst precursor with a combined streamof the lower alkane methane and inert diluent gas like nitrogen at aconstantly increasing temperature from e.g. 100 to 750° C. remarkablyimproves the stability/performance of the catalyst for methanearomatization. Catalyst performance is even further improved in case thecatalyst precursor is pre-carburized under the combined stream ofmethane and nitrogen at a constantly increasing temperature to thetemperature useful for aromatization and is subsequently kept for e.g.15 minutes at the temperature useful for aromatization; see Tables 6-7.

The invention claimed is:
 1. A zeolite catalyst useful for aromatizationof a lower alkane obtainable by the method comprising: contacting amedium pore zeolite catalyst precursor with a pre-carburizing gas streamcomprising a pre-carburizing gas stream lower alkane and 50-90 more-% ofan inert diluent gas at a temperature that is increased from 20-250°C./minute or less to a temperature useful for aromatization and keepingthe temperature constant for 0-60 minutes at the temperature useful foraromatization to produce the zeolite catalyst; wherein the zeolitecatalyst precursor comprises 2-10 wt % molybdenum (Mo) and 0-2 wt % ofan additional element selected both Groups 6-11 of the Periodic Table;wherein the zeolite is de-aluminated and has a Si/Al ratio of 10-50. 2.The zeolite catalyst of claim 1, wherein the zeolite catalyst precursoris produced by the process comprising: (i) contacting a zeolite with asolution comprising molybdenum (Mo) and optionally a solution comprisingthe additional element selected from Group 6-11 of the Periodic Table;and (ii) drying and calcining the zeolite to provide a zeolite catalystprecursor.
 3. The zeolite catalyst of claim 1, wherein the temperatureis kept constant for 5-60 minutes at the temperature useful foraromatization after attaining said temperature useful for aromatization.4. The zeolite catalyst of claim 1, further comprising, subsequent tokeeping the temperature constant for 0-60 minutes at the temperatureuseful for aromatization, contacting the zeolite catalyst with afeedstream lower alkane, wherein the lower alkane is methane (CH₄),ethane (C₂H₆) or a mixture thereof.
 5. The zeolite catalyst of claim 1,wherein the inert diluent gas is selected from the group consisting ofnitrogen (N₂), helium (He) and argon (Ar).
 6. A process foraromatization of a lower alkane comprising contacting the catalystaccording to claim 1 with a feedstream comprising a lower alkane atconditions useful for aromatization.
 7. The process of claim 6, whereinthe temperature useful for aromatization is 700° C.-750° C.
 8. Theprocess of claim 6, wherein the pre-carburizing gas stream consists of alower alkane and an inert diluent gas.
 9. The process claim 6, whereinthe lower alkane is methane (CH₄), ethane (C₂H₆) or a mixture thereof.10. A zeolite catalyst useful for aromatization of a lower alkaneobtainable by the method comprising: contacting a medium pore zeolitecatalyst precursor with a pre-carburizing gas stream comprising apre-carburizing gas stream lower alkane and 50-90 mole-% of an inertdiluent gas at a temperature that is increased from 20-250° C. at a rateof about 20° C./minute or less to a temperature useful for aromatizationand keeping the temperature constant for 0-60 minutes at the temperatureuseful for aromatization to produce the zeolite catalyst; wherein thezeolite catalyst precursor comprises 2-10 wt % molybdenum (Mo) and 0-2wt % of an additional element selected from Groups 6-11 of the PeriodicTable; wherein the zeolite is de-aluminated.
 11. The zeolite catalyst ofclaim 10, wherein the zeolite catalyst precursor is produced by theprocess comprising: (i) contacting a zeolite with a solution comprisingmolybdenum (Mo) and optionally a solution comprising the additionalelement selected from Group 6-11 of the Periodic Table; and (ii) dryingand calcining the zeolite to provide a zeolite catalyst precursor. 12.The zeolite catalyst of claim 10, wherein the temperature is keptconstant for 5-60 minutes at the temperature useful for aromatizationafter attaining said temperature useful for aromatization.
 13. Thezeolite catalyst of claim 10, wherein the zeolite catalyst precursorfurther comprises a binder.
 14. The zeolite catalyst of claim 10,further comprising, subsequent to keeping the temperature constant for0-60 minutes at the temperature useful for aromatization, contacting thezeolite catalyst with a feedstream lower alkane, wherein the loweralkane is methane (CH₄), ethane (C₂H₆) or a mixture thereof.
 15. Thezeolite catalyst of claim 10, wherein the inert diluent gas is selectedfrom the group consisting of nitrogen (N₂), helium (He) and argon (Ar).16. A process for aromatization of a lower alkane comprising contactingthe catalyst according to claim 10 with a feedstream comprising a loweralkane at conditions useful for aromatization.
 17. The process of claim16, wherein the temperature useful for aromatization is 600° C.-850° C.18. The process of claim 16, wherein the temperature useful foraromatization is 700° C.-750° C.
 19. The process of claim 16, whereinthe pre-carburizing gas stream consists of a lower alkane and an inertdiluent gas.
 20. The process claim 16, wherein the lower alkane ismethane (CH₄), ethane (C₂H₆) or a mixture thereof.
 21. The process ofclaim 16, wherein the inert diluent gas is selected from the groupconsisting of nitrogen (N₂), helium (He) and argon (Ar).